Effects of artificially controlling levels of Lake Taupo, North Island, New Zealand, on the Stump Bay wetland

Abstract

The Stump Bay wetland is part of the South Taupo wetland, situated on the southern coast of Lake Taupo in the central North Island, New Zealand. The water level of Lake Taupo has been artificially controlled since 1941. The present study found that surface and subsurface water levels in the wetland are closely related to each other. These water levels are influenced mainly by rainfall and evapotranspiration (which is seasonal) in areas that are greater than c. 100 m landward from the lake edge, but by the lake level for areas in close proximity to the lake. The wetland is a discharge and/or through‐flow area, and the general flow direction is towards the lake. The time lag for a groundwater rise in the eastern part of the wetland after rainfall was found to be 24 h. The artificial regulation of the lake level has led to a 2 cm rise in annual mean lake levels since 1950. However, when means are calculated for each month separately, the artificially controlled lake level is up to 20 cm higher in the summer than the pre‐controlled summer levels. Significantly higher summer lake levels since 1941 are likely to have caused the observed increase in water levels in the Stump Bay wetland area. Further increases in lake levels are likely to decrease areas of native shrub communities and encourage spreading of the exotic shrub Salix cinerea. Lowering in lake levels may decrease areas of native low‐stature vegetation, but the spreading of native shrub communities may be curbed by competing S. cinerea.